1. Field of the Invention
The present invention is directed generally to an electrochemical gas sensor, and more particularly to an electrochemical gas sensor in which a portion of the components are formed using microelectromechanical systems technology.
2. Background of the Invention
Electrochemical gas sensors are typically employed in monitoring equipment, such as in equipment used in medical applications, to measure the concentration of a particular gas in a gas sample. Such equipment typically includes a display to indicate numerical readings of gas concentrations and typically provides output waveforms corresponding to the gas concentrations.
FIG. 1 illustrates a typical electrochemical gas sensor 10 of the relevant art. The sensor 10 includes a housing 12 which contains the components of the sensor 10. A sensing electrode 14 may be constructed of, for example, a noble metal such as silver. A counter electrode 16 may be constructed of, for example, lead. An electrolyte 18 such as, for example, an aqueous solution of potassium hydroxide, fills the housing 12. Together, the sensing electrode 14, counter electrode 16, and electrolyte 18 form an electrochemical cell. An expansion membrane 20 allows for expansion and contraction of the electrolyte 18 without damaging the sensor 10. A diffusion barrier 22, such as a membrane made of fluoropolymer resin sold under the trade name Teflon.RTM., a registered trademark of E.I. Du Pont de Nemours and Company, is adjacent the sensing electrode 14, and limits the diffusion rate of the gas to be measured by the sensor 10.
Typical relevant art sensors 10 are manufactured serially. That is, the sensors 10 are manufactured from different and discrete components according to many assembly and sealing processing steps. Thus, there is little cost benefit in manufacturing sensors 10 in high volume quantities. In addition, conventional sensors are often relatively large, about ten cubic centimeters, making them too intrusive for many applications.
The performance of relevant art sensors 10 is also limited by the characteristics of the discrete components of the sensor 10, as well as the required assembly process. The diffusion barrier 22 of the sensor 10 limits the capability of the sensor 10 to monitor rapid changes in gas concentrations: the thicker the diffusion barrier 22, the slower the response time of the sensor 10. Typical relevant art sensors 10 have a diffusion barrier 22 of at least five to six microns. A typical response time for such a relevant art sensor 10 is approximately 500 ms. Such response times may not be acceptable for many applications. Moreover, the minimum thickness of the diffusion barrier 22 is limited to the availability of materials from commercial suppliers and the handling requirements during conventional sensor assembly. Thus, the response times of relevant art electrochemical gas sensors are limited to values which may not be fast enough for some applications.
In addition, typical relevant art sensors 10 are temperature and pressure dependent, and do not allow for integration of electrical systems to compensate for the effects of temperature and pressure.
Accordingly, there exists a need in the relevant art for an electrochemical gas sensor which is less expensive to produce and which is smaller in size. There also exists a need for an electrochemical gas sensor which realizes faster response times than relevant art sensors in response to rapid changes in the concentration of the gas to be measured. There also exists a need for an electrochemical gas sensor which allows for the integration of other sensing elements and electronic circuits.